The invention relates to the field of spintronics and, in particular, to graphene-(antiferro)-ferromagnet multilayers (GMMs) useful in spintronic devices.
Progress in the miniaturization of traditional solid state electronics based on manipulating electronic charges is rapidly approaching the natural technological limit imposed by the discrete atomic structure of matter. Already working at the nanometer scale, current technology not only encounters significant challenges requiring ever-increasing nanoscience research and design effort, but after a more than six order-of-magnitude reduction in size of the electronic components achieved by the beginning of this century, there is less than an order of magnitude left to go. Consequently, there has been a growing recognition that one of the avenues for future progress rests with a new approach in electronics, dubbed spintronics, where not only electron's charge, but also its quantum spin degree of freedom is manipulated.
The first application of a spintronic effect, giant magnetoresistance or GMR, can be found in magnetic sensing devices. GMR technology uses one type of electron spin manipulation, the change in resistance of a resistive element in the presence of a magnetic field. GMR elements can therefore be considered spintronic analogues of conventional resistors. These devices have been applied to magnetic read heads for computer hard drives and magnetic random access memory (MRAM). They also garnered the 2007 Nobel Prize in Physics for discoverers Albert Fert and Peter Grünberg.
A number of materials, including natural half-metals, such as chromium dioxide, doped perovskite manganites, and various magnetic semiconductors have been investigated as possible sources of spin-polarized electrons for spintronic devices.
At present, available devices sensitive to the polarization of electric current utilize the GMR-dependence of the resistance on the spin orientation in alternating ferromagnetic and antiferromagnetic or magnetic and nonmagnetic multilayers, e.g. in Iron/Chromium/Iron (Fe/Cr/Fe) trilayers. For a Cr interlayer of appropriate thickness, the coupling between the adjacent ferromagnetic iron layers is antiferromagnetic, and in the absence of an external magnetic field their magnetizations are antiparallel. An external magnetic field can co-align magnetization of magnetic layers and this decreases the amount of spin-dependent electron scattering, decreasing the resistance. At present, approximately 90% of all hard drive read heads use the GMR technology which allows the storage density to be increased by over two orders of magnitude compared to earlier technology.
Other approaches to producing spin-polarized electric currents are based on magnetically doped semiconductors, such as (Ga1-xMnx)As, (In1-xMnx)As, (Be1-xMnxZny)Se, Cd1-xMnxTe, etc., where the properties of ferromagnetic and semiconducting systems are combined on the material level. Both semiconducting properties and carrier-controlled magnetism of these materials are mediated by the same doped charge carriers. Hence, there exists not only direct coupling between the two, but also an intricate interplay between magnetic and semiconducting properties, requiring a fine doping/compositional optimization, which has hindered technological progress so far. Similar material chemistry problems are creating obstacles on the path of using natural semimetals with spin-polarized electronic bands, such as Fe3O4, or La0.7Sr0.3MnO3.